June 9, 1898] 



NATURE 



137 



resins, but also very largely upon the amount and the character 

 of the impurities contained in the material. 



The specific gravity of cleaned gutta percha of average com- 

 position is very nearly the same as that of water, but that of 

 individual brands deviates considerably from it, some being about 

 3 per cent, lighter, and others about 2 per cent, heavier, as 

 will be seen from Table I. , which gives the specific gravities for 

 eleven definite brands and an average material obtained by 

 mixing a number of different cleaned materials in the masticator. 

 The table also gives for comparison the specific gravities of 

 balata, of gutta percha extracted from leaves with petroleum 

 spirit by Dr. Obach's patent process, and also of pure Para- 

 caoutchouc. 



The exceptionally low specific gravity of the gutta percha from 

 leaves is to be attributed to the fact that it consists almost 

 entirely of pure gutta. 



Table I.— Specific Gravity of Cleaned Gutta Percha. 

 (2 "2 mm. sheet.) 



Group 



Genuine 



II. 



Soondie 



III. 



White 



IV. 

 Mixed 



V. 

 Various 



Name of brand 



Pahang 

 Banjer red ... 

 Bulongan red 



Spec. grav. 

 at 15° C. 



0-9858 

 09868 

 0-9911 



Bagan 



Kotaringin 



Serapong 



Bulongan 



Mixed 



Padang 



Padang reboiled ... 

 Sarawak mixed 

 Mixed after cleaning 



Balata 



G.P. from leaves 



Para-caoutchouc 



0-9709 

 0-9729 

 0-9767 



1-0093 

 I -0186 

 0-9911 



0-9960 

 0-9912 

 I 0022 



0-9731 

 0-9625 

 0-9275 



Ratio 

 G 

 R 



4-0 



1-44 

 1-30 

 1-38 



1-57 

 I-I4 

 1-40 



1-18 

 I -20 

 175 



i-i6 

 51-90* 



• Not 519, as erroneously stated in ih^ Journal and Reprint. 



The absorption of water by gutta percha was ascertained by 

 immersing strips of the cleaned material in water and weighing 

 them at regular intervals for about eighteen weeks. The results 

 of these tests made on representative materials of the four 

 groups which have been mentioned, and on gutta obtained 

 from leaves, on balata and caoutchouc, are graphically shown in 

 Fig. I. 



The curves shown on the left of the diagram (Fig. i ) represent 

 the average results obtained for the different brands composing 

 the various groups or " classes," as well as the results for gutta 

 percha extracted from leaves, for balata, and also for pure Para- 

 caoutchouc ; but the curves on the right of the diagram were in- 

 directly obtained by calculation and represent the absorption, 

 which would have taken place if each specimen tested had 

 entirely consisted of the kind of gutta which is characteristic of 

 it. The reason for this reduction of the results to " pure gutta " 

 is that the water is exclusively absorbed by this component and 

 not by the resin, which has been found impervious to it. 



It will be seen from the diagram, that it is the group of 

 "genuine" materials which absorbed the most water, both before 

 and after the reduction to "pure gutta"; whereas of the 

 materials as such, it is the " white " sorts which are the least 

 permeable to water, and of the "pure guttas " that of the material 

 extracted from leaves. 



Pure Para-caoutchouc, as is generally known, has a considerably 

 greater absorptive power for water than even the most permeable 

 kind of gutta percha. 



NO. 1493, VOL. 58] 



The temperature at which gutta percha becomes plastic, a 

 physical property of practical importance, depends almost entirely 

 upon the relative proportion of gutta and resin. The great 

 difference existing in that respect between the different sorts was 

 demonstrated in the lecture by an experiment illustrated in 

 diagram (Fig. 2), 



The apparatus consists of a frame,/, holding three strips of 

 gutta percha, i, 2, 3, under the tension of springs s, Sj S3, the 

 frame is lowered into a beaker of water, v, and the latter slowly 

 heated, the arrangement being such that an electric current is 

 established, and an alarum. A, sounded as soon as a strip becomes 

 sufficiently soft to allow the spring to pull it apart. The three 

 materials employed contained 2^, 38 and 60 per cent, of resin, 

 and the temperatures at which they softened were found to be 

 55", 48° and 42° C. respectively. 



Another physical property, viz. the time required by gutta 

 percha to harden or set again on cooling, after having previously 

 been softened by heat, also depends mainly on the relative 

 percentage of gutta and resin, as was pointed out by the 

 lecturer. 



Caoutchouc ^o-^o^o 



Fig. I. -^Absorption of water by different " classes" of gutta percha. 

 (Thickness of sheet 2*2 mm. ; are.-i, i sq. dm. ; weight, to g.) 



The mechanical properties of gutta percha, of which the tensile 

 strength is the principal one, are in their turn also greatly 

 affected by the percentage of lesin. 



The important electrical properties of gutta percha chiefly 

 depend on the nature of the gutta, and, to a lesser extent, on that 

 of the resin, but only slightly on the relative proportion of these 

 two components. 



The insulating property of gutta percha was stated to have 

 been first observed by Dr. Werner von Siemens in 1846. Fara- 

 day also noticed it shortly afterwards, and called attention to it 

 in March 1848. 



Dr. Obach showed the two principal electrical properties by 

 means of an electroscope arranged as shown in Fig. 3. The 

 instrument was provided with a flat brass disc, P, dt the top, and 

 below it two pith rods, /, /, were suspended on either side of a 

 fixed strip of brass, m. When a piece of gutta p)ercha tissue 

 was spread over the brass disc and the electroscope charged by 

 means of the brass knob, k, at the side, the pith rods diverged 

 and remained stationary. If the fingers were now placed on 

 the covered disc, the rods slightly converged and then again 



